Solids inventory is a critical parameter for catalytic reactors: it impacts the geometrical configuration of the reactor, compressor/blower requirements, and the overall economics of the process. Although significant advances have been made for characterizing the hydrodynamic regimes in gas-solid fluidization, empirical relationships for predicting the solids inventory in risers are largely in the developmental stages. Empirical models are either based on the slip velocity, slip factor or the Richardson - Zaki bed expansion relationship. We propose the drift flux model, commonly for gas-liquid hydrodynamic modelling. The drift flux model accounts for radial variations in the axial velocity profile and void fraction with a distribution parameter, C-0, and the relative velocity between the phases with the drift velocity, U-2j: =U-g/C-0(U-g+U-p)+(epsilon V-gj)/ In high density risers, the distribution parameter varies between 1.03 and 1.12. The drift velocity is a function of the product of the particle terminal velocity and varies with solids hold-up.